1. Field of the Invention
This invention relates generally to a method of growth of Group II metal-containing films, e.g., films of barium strontium titanate (BST) and other Group II metal (Ba, Sr, Ca, Mg) oxide films, using polyamine precursors. The invention contemplates novel precursors of such type.
2. Description of the Related Art
Many materials are utilized in the form of thin films on substrates and are formed by vapor deposition techniques. Examples include refractory materials such as high temperature superconducting (HTSC) materials including YBa.sub.2 CU.sub.3 O.sub.x, wherein x is from about 6 to 7.3, BiSrCaCuO, and TlBaCaCuO. Barium titanate, BaTiO.sub.3, and barium strontium titanate, Ba.sub.x Sr.sub.1-x TiO.sub.3, have been identified as ferroelectric and photonic materials with unique and potentially very useful properties in thin film applications of such materials. Ba.sub.x Sr.sub.1-x Nb.sub.2 O.sub.6 is a photonic material whose index of refraction changes as a function of electric field and also as a function of the intensity of radiated light. The Group II metal fluorides, BaF.sub.2, CaF.sub.2, and SrF.sub.2, are useful for scintillation detecting and coating of optical fibers.
Many of the potential applications of these materials require their use in thin film, coating, or layer form. The films or layers may also be advantageously epitaxially related to the substrate upon which they are formed. Applications in which materials may need to be deposited in film or layer form include integrated circuits, switches, radiation detectors, thin film capacitors, holographic storage media, and various other microelectronic optical and communication devices.
Chemical vapor deposition (CVD) is a particularly attractive method for forming these layers because it is readily scaled up for production. Further, the electronic industry has extensive experience and an established CVD equipment base that can be applied to new CVD processes. In general, the control of key variables such as stoichiometry and film thickness, and the coating of a wide variety of substrate geometries is possible with CVD.
Forming the thin films by CVD will permit the integration of these materials into existing device production technologies. CVD also permits the formation of layers of materials that are epitaxially grown on substrates having close crystal structures and lattice parameters.
CVD requires that the element source reagents, i.e., the precursor compounds and complexes containing the elements or components of interest, must be sufficiently volatile to permit gas phase transport into the chemical vapor deposition reactor. The elemental component source reagent must decompose in the CVD reactor to deposit only the desired element at the desired growth temperatures. Premature gas phase reactions leading to particulate formation must not occur, nor should the source reagent decompose in the lines before reaching the reactor deposition chamber. When compounds are decomposed for deposition, obtaining optimal properties requires close control of stoichiometry which can only be achieved if the reagent can be delivered into the reactor in a controllable fashion. In this respect the reagents must not be so chemically stable that they are non-reactive in the deposition chamber.
Desirable CVD reagents, therefore, are fairly reactive and volatile. Unfortunately, for many of the materials described above, volatile reagents do not exist. Many potentially highly useful refractory materials have in common that one or more of their components are Group II elements, i.e., the metals barium, calcium, strontium, or magnesium, for which no or few volatile compounds well-suited for CVD are known. In many cases, the source reagents are solids whose sublimation temperature is very close to the decomposition temperature, in which case the reagent may begin to decompose in the lines before transport to the reactor, and it therefore is very difficult to control the stoichiometry of the deposited films from such decomposition-susceptible reagents.
In other cases, the CVD reagents are liquids, but their delivery into the CVD reactor in the vapor phase has proven impractical because of problems of premature decomposition or stoichiometric control.
When the film being deposited by CVD is a multicomponent substance rather than a pure element, such as barium titanate or the oxide superconductors, controlling the stoichiometry of the film is critical to obtaining the desired film properties, such as for example optical or electrical properties. In the deposition of such materials, which may form films with a wide range of stoichiometries, the controlled delivery of known proportions of the source reagents into the CVD reactor chamber is essential.
While source reagent liquid delivery systems present distinct advantages over conventional techniques, there is often some fraction of the precursor compound that decomposes into very low volatility compounds that remain in the vaporization zone. This deficiency is an important issue in the operation of CVD processes that use thermally unstable solid source precursors which undergo significant decomposition at conditions needed for sublimation. Such decomposition can occur in all reagent delivery systems that involve a vaporization step, including flash vaporizer liquid delivery systems, as well as more conventional reagent delivery systems that include bubblers and heated vessels operated without carrier gas(es).
Although well-behaved CVD precursors vaporized under "ideal" conditions will form no deposits or residue at the vaporization zone, deviations from this situation are common and can be divided into several categories:
1) Reactive impurities in either the precursor or in the carrier gas decompose at the vaporizer temperatures.
2) Spatial and temporal temperature variations occur in the vaporization zone, with temperatures in some regions being sufficient to facilitate decomposition.
3) CVD precursors are employed which are thermally unstable at the sublimation temperature.
Optimization of the conditions used in the vaporizer of reagent delivery systems can minimize the fraction of the delivered precursor that decomposes (and remains) in the vaporization zone, but virtually all solid and liquid precursors undergo some decomposition when they are heated for transport to the gas phase. This fraction is negligibly small in "well-behaved" precursor compounds. Use of precursors that tend to decompose near their vaporization temperature may be mandated by availability (i.e., where the selected precursor possesses the best properties of available choices) or by economics, where precursor cost is strongly dependent on the complexity of its synthesis and molecular structure.
Additionally, CVD precursors often contain impurities, and the presence of those impurities can cause undesirable thermally activated chemical reactions in the vaporization zone, also resulting in the formation of involatile solids, liquids and oligomers/polymers at that location. For example, a variety of CVD precursors (such as tantalum pentaethoxide) are water-sensitive and hydrolysis can occur at the heated vaporizer zone forming tantalum oxide particulates that may be incorporated into the growing tantalum oxide film with deleterious effects or induce problems in the delivery or vaporization zones.
Despite the advantages of the liquid delivery approach (which include improved precision and accuracy for most liquid and solid CVD precursors and higher delivery rates), the foregoing deficiencies pose a serious impediment to widespread use of the vaporization liquid delivery technique for providing volatilized reagent to the CVD reactor and full-scale manufacturing of electronic components.
The foregoing problems have specifically been experienced in the development of high density memories using high dielectric constant and ferroelectric materials. In addition to high density memories, ferroelectric materials are attractive candidates in a wide variety of solid state sensors and imaging devices, as a consequence of their pyroelectric and piezoelectric properties. Production worthy deposition modules are needed to realize the potential of high dielectric constant and ferroelectric materials in evolving semiconductors. The preferred method for production of films of these materials is MOCVD, but at present a full complement of stable liquid source reagents is not commercially available for many thins films of interest, such as BaSrTiO.sub.3 (BST) and SrBi.sub.2 Ta.sub.2 O.sub.9 (SBT).
The vaporization of solid Group II source reagents such as those used in the MOCVD of these materials presents additional difficulties. First, non-fluorinated source reagents may undergo some decomposition during vaporization. Oligomerization or polymerization may accompany decomposition and cause the effective transport rate of the source reagent to drop much more rapidly than expected based on the amount of decomposed material. This phenomenon is not reproducible and is one of the reasons that many groups have designed their CVD tools to allow a fresh charge of precursor(s) before each run. See, for example, J. M. Zhang, J. Zhao, H. O. Marcy, L. M. Tonge, B. M. Wessels, T. J. Marks, and C. R. Kannewurf, Appl. Phys. Lett., 54, 1166 (1989). Fluorinated precursors are undesirable because fluorine is an anionic dopant which requires temperatures in excess of 800.degree. C. in the presence of water to remove it. Fluorinated precursor processes are potentially destructive for ULSI devices, particularly the storage node, and measured leakage currents of MOCVD films grown with fluorinated precursor reagents at temperatures below 700.degree. C. have been unacceptable. The use of a liquid delivery system overcomes some of these problems, but does not completely eliminate decomposition residue (in every case) during flash vaporization.
Second, the Group II source reagent materials do not have high vapor pressures, so all sections of the reactor between the vaporization point and any trap used to remove undecomposed precursor before the vacuum pump must be heated. This brings considerable added complexity and expense to the reactor design. In particular, cost and complexity rise steeply with increase of required temperatures from around 180.degree. C. to 240.degree. C. This is because elastomer vacuum seals cannot withstand temperatures above the 200-220.degree. C. range, and therefore metal seals must be used. The development of precursors which can be vaporized and will not condense on reactor walls at lower temperatures would entail significant commercial advantages for the commerical production of Group II element-containing films such as BST.
Improved liquid delivery systems are disclosed in U.S. Pat. No. 5,204,314 issued Apr. 20, 1993 to Peter S. Kirlin et al. and U.S. Pat. No. 5,536,323 issued Jul. 16, 1996 to Peter S. Kirlin et al., which describe heated foraminous vaporization structures such as microporous disk elements. In use, liquid source reagent compositions are flowed onto the foraminous vaporization structure for flash vaporization. Vapor thereby is produced for transport to the deposition zone, e.g., a CVD reactor. The liquid delivery systems of these patents provide high efficiency generation of vapor from which films may be grown on substrates. Such liquid delivery systems are usefully employed for generation of multicomponent vapors from corresponding liquid reagent solutions containing one or more precursors as solutes, or alternatively from liquid reagent suspensions containing one or more precursors as soluble suspensions.
The art continues to seek improvements in liquid delivery systems for vapor-phase formation of advanced materials, as well as improvements in process conditions and techniques for operating the liquid delivery system and ancillary equipment in a maximally efficient manner.
Accordingly, it is an object of the present invention to provide new Group II precursor compositions which are usefully employed in liquid delivery MOCVD processes for the formation of Group II metal-containing films.
It is another object of the invention to provide new precursor compositions for the formation of ferroelectric films, such as SBT and high dielectric constant materials, such as BST.
It is a still further object of the invention to provide an efficient liquid delivery process for the formation of Group II metal-containing films such as BST films.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.